plawolf
Lieutenant General
If they designed it right then optimally it should be whatever the cal and above that can threaten the integrity of the base armour to which the ERA is applied on.
Not that I don't trust you, but do you have any sources for that? As I think it would be interesting reading.
Also, if you are right, it kind of raises another question - how effective is ERA after it has already been perforated by low cal weapons, like AMRs, HMGs, 20-30mm etc?